How to remove unwanted edge between adjacent coplanar faces?
The 'meaningless' edges are not meaningless.



because, internally ..


A face is bounded by a single unidirectional loop of edges .. so..
An edge may not appear twice in the same loop


The red areas in first two examples above are not faces, as far as Blender is concerned. The last example qualifies to be 2 faces.

So in this case, Blender has held on to the minimum number of edges to preserve the faces around the frame of your crate. If you got rid one or both of those edges, (say, by deleting them ) the faces would have to go too.